SPRINGERVILLE SOLAR FARM

The state of Arizona is well positioned to benefit from growth in solar energy generation and consumption. Arizona’s relatively close proximity to the mid-west and power-hungry California combined with an abundance of sunny days makes it an ideal location for the profitable generation of solar power.

There are two conventional solar technologies currently in large scale use. The solar thermal method typically involves large arrays of concentrating reflectors which are used to heat a working fluid. This heat is used by turbine engines to operate generators in a conventional manner. Such solar generating plants require cooling towers that use large quantities of water, are mechanically complex and require the same level of operational staffing and expertise as fossil fuel generating plants. Nevada's Solar One, built by Acciona Solar Power (formerly Solargenix) is an example of this type of solar generating facility.

The second conventional solar technology is Photovoltaics (PV). PV uses arrays that convert sunlight directly into electricity, bypassing the heat conversion step altogether. Because of the arid nature of this region, photovoltaics offer a low impact method of constructing large-scale solar farms.

One such plant, the Springerville Arizona Generating Station built for Tuscon Electric Power Company (TEP) by Global Solar Energy Incorporated, has a continuous PVUSA (Photovoltaics for Utility Scale Applications) generating capacity of 3812 AC kW in 2007. (3.8 AC Megawatts)

 
Total Array Coverage Area  44 Acres 
Number of PV Arrays - Fixed at 34 Degrees Tilt 34 Arrays
Fixed Array Azimuth Direction 180 Degrees - South
Total Number of PV Modules 34,980 modules
Total PV DC Array Capacity Rating (STC) 4,590 kW
Total Inverter AC Capacity Rating 5,100 kW
Total System AC PVUSA Rating 3,812 kW
System Actual AC Output - 1 Minute Rating 5,113 kW (Dec 6, 2004)
System Actual AC Output - 15 Minute Rating 4,644 kW
Actual 2006 Annual Net AC Energy Production 7,765 MWh
Reliability in 2003/2004/2005/2006 99.78%/99.72%/99.81%/99.75%

Site preparation was performed in a way that disturbed native vegetation as little as possible. A water permeable dust/erosion control agent was placed on the soil after leveling to promote growth of the native grasses. Growth of weeds is being controlled by manual removal until such time as the native grasses reestablish themselves as the dominant species. Native grasses generally reach a full growth height of about 8 inches and the lower edge of the PV modules are therefore placed 8 inches above the ground. The modules are fixed, south facing with a latitude tilt of 34 degrees from the horizontal. The area is fenced to keep out cattle. Lizards and small mammals moved back into the area after construction and use the PV modules for shade.

The system operates as an unmanned site and is monitored continuously over the Internet . Nearly all operational functions can be performed by remote control. Data is taken from the inverters and revenue meters in 10 second scan cycles and averaged over 1 minute. Spare parts are maintained on site and local service personnel are dispatched to perform repairs as required.

The local distribution line is capable of supporting 36 MW of distributed generation.The power plant produces the most capacity during the cooler months of the year when the sun is near the latitude angle of the arrays. This typically occurs in March and April. Solar insolation at the 6650 foot elevation site has been measured at over 1,500 watts per square meter in these months for short periods and at over 1,300 for one minute averages.

Conventional solar power stations are affected by clouds. Clouds usually decrease the available solar energy but they can also have the opposite effect. Cloud passage creates strong sunsplash conditions. Sunsplash is the effect caused when the sun moves into a gap between clouds. Solar panels will get full direct sunlight and additional reflected light from the clouds themselves. This results in a momentary increase of output from the solar arrays. The actual AC output of the Springerville power station during severe sunsplash has been measured at 157 kW for 10 second averages. Consequently, the short term power output rating of the system with 34 units installed is an actual 5,113 kW measured for a one minute average. (December 6th, 2004)

It is important to note that there are two types of tests used to rate the output of solar panels: (1) STC (Standard Test Conditions) – the solar panel's output is tested in a lab and the output stated by the manufacturer (2) PTC (PVUSA Test Conditions) – the solar panel's output it tested in real world conditions at the PVUSA testing center in Davis, California. The PTC rating is typically 10-15% lower than the STC rating.

The impact of dust and other debris (referred to as soiling) on PV output is a widely discussed and frequently misunderstood issue. The Springerville solar plant operates satifactorily without any array cleaning other than normal rainfall and other natural agents such as wind.

Designers commonly estimate that soiling will reduce annual module output by 1% to 4%. Bird droppings, pollution, and dust from traffic or farming activities can reduce output by as much as 20% over the course of a dry summer. Other variables -- such as surface material and orientation -- are also believed to influence soiling.

PVUSA conducted a simple side-by-side test of two identical modules installed on a fixed-tilt rooftop. One module was cleaned three times a week while the other was left to the forces of nature. PVUSA has determined annualized soiling losses can be expected to exceed 7% during a normal rainfall year, but only 4% during a wet year. PVUSA estimates that during drought years the annual soiling losses may exceed 10%. These results suggest that PV systems would benefit from array cleaning. ("How Clean is My Array? The Real Dirt on Soiling,"PVUSA Project Update, Third Quarter 1999)